In recent years, surface-treated steel sheets made by imparting rust resistance to base steel sheets, particularly galvanized steel sheet and galvannealed steel sheets which can be manufactured at low cost and which have excellent rust resistance, have been used in fields such as automobiles, home appliances, and building materials. In view of the improvement of automotive fuel efficiency and the improvement of automotive crash safety, there are increasing demands for lightweight high-strength automobile bodies using automobile body materials having high strength and a reduced thickness. Therefore, high-strength steel sheets are increasingly used for automobiles.
In general, galvanized steel sheets are manufactured in such a manner that thin steel sheets which are prepared by hot-rolling and cold-rolling slabs and which are used as base members are subjected to recrystallization annealing and galvanizing in a continuous galvanizing line (hereinafter also referred to as CGL) including an annealing furnace. Galvannealed steel sheets are manufactured in such a manner that the thin steel sheets are further subjected to alloying subsequently to galvanizing.
Examples of the type of the annealing furnace of the CGL include a DFF (direct fired furnace) type, a NOF (non-oxidizing furnace) type, and an all-radiant tube type. In recent years, CGLs including all-radiant tube-type furnaces have been increasingly constructed because the CGLs are readily operated and are capable of manufacturing high-quality plated steel sheets at low cost due to rarely occurring pick-up. Unlike DFFs (direct fired furnaces) and NOFs (non-oxidizing furnaces), the all-radiant tube-type furnaces have no oxidizing step just before annealing and therefore are disadvantageous in ensuring the platability of steel sheets containing oxidizable elements such as Si and Mn.
PTLs 1 and 2 disclose a method for manufacturing a hot-dipped steel sheet including a base member that is a high-strength steel sheet containing a large amount of Si and Mn. In the method, the heating temperature in a reducing furnace is determined by a formula relating the partial pressure of steam and the dew point is increased such that a surface layer of the base member is internally oxidized. The presence of internal oxides is likely to cause cracking during machining, thereby causing a reduction in anti-powdering property. A reduction in corrosion resistance is also caused.
PTL 3 discloses a technique for improving coating appearance in such a manner that not only the concentrations of H2O and O2, which act as oxidizing gases, but also the concentration of CO2 are determined such that a surface layer of a base member just before being plated is internally oxidized and is inhibited from being externally oxidized. In the technique disclosed in PTL 3 as well as PTLs 1 and 2, the presence of internal oxides is likely to cause cracking during machining, thereby causing a reduction in anti-powdering property. A reduction in corrosion resistance is also caused. Furthermore, there is a concern that CO2 causes problems such as furnace contamination and changes in mechanical properties due to the carburization of steel sheets.
Recently, high-strength galvanized steel sheets and high-strength galvannealed steel sheets are increasingly used for parts difficult to machine; hence, anti-powdering property during heavy machining becomes important. In particular, in the case of bending a plated steel sheet to more than 90 degrees such that the plated steel sheet forms an acute angle or in the case of machining the plated steel sheet by impact, a coating on a machined portion thereof needs to be inhibited from being peeled off.
In order to satisfy such a property, it is necessary to achieve a desired steel microstructure by adding a large amount of Si to steel and it is also necessary to highly control the microstructure and texture of a surface layer of a base steel sheet that lies directly under a plating layer which may crack during heavy machining. However, such control is difficult for conventional techniques; hence, it has been impossible to manufacture a galvanized steel sheet which has excellent anti-powdering property during heavy machining and which includes a base member that is a Si-containing high-strength steel sheet using a CGL including an annealing furnace that is an all-radiant tube-type furnace.